1. Field of the Invention
This invention relates to a process for producing a ferromagnetic iron oxide powder useful for magnetic recording, and more specifically, to a process for producing a ferromagnetic iron oxide powder having a high coercive force, a superior stability to pressure and heat and a high degree of dispersibility in organic solvents.
2. Description of the Prior Art
The ferromagnetic iron oxide for high density magnetic recording should desirably have a high coercive force.
Known techniques for increasing the coercive force of a ferromagnetic iron oxide powder include, for example, a method which comprises causing the ferromagnetic iron oxide particles found to be acicular and increasing the shape anisotropy of the ferromagnetic iron oxide particles, a method which involves adding cobalt to the ferromagnetic iron oxide, and a method for producing acicular ferromagnetic iron oxide containing cobalt which has the advantages of the first two methods.
One typical method for adding cobalt to acicular ferromagnetic iron oxide comprises incorporating cobalt ion in .alpha.-FeOOH, .gamma.-FeOOH, .beta.-FeOOH, etc. which are starting materials for the production of ferromagnetic iron oxide, and calcining the mixture to form a ferromagnetic powder.
Another method for obtaining cobalt-containing a acicular ferromagnetic iron oxide comprises adhering a cobalt compound to the surface of cobalt-free ferromagnetic iron oxide, and then calcining the product.
Magnetic recording media such as magnetic tapes prepared using cobalt-containing ferromagnetic iron oxide obtained by the methods described above have the defect that they are unstable to pressure and heat, and on repeated reproduction, the magnetic signals recorded become weak, or print through is great.
Another method for increasing the coercive force of ferromagnetic iron oxide using cobalt comprises adhering a cobalt-containing layer intimately to the surface of acicular ferromagnetic iron oxide, and increasing the adhered layer (see, for example, Japanese Patent Publication No. 4825/62 and U.S. Pat. No. 3,046,158). This technique has been improved as described in Japanese Patent Publication No. 49475/74, Japanese Patent Application (OPI) No. 113199/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application", hereinafter the same) (corresponding to British Pat. No. 1,441,183), and Japanese Patent Publication Nos. 29157/75 and 33319/77.
In the method comprising increasing a cobalt ferrite layer as described above, the coercive force of the ferromagnetic iron oxide is nearly proportional to the volume of the cobalt ferrite layer on the surface of the iron oxide. The cobalt ferrite layer comprises cobalt doped ferromagnetic iron oxide which is unstable to pressure and heat. Therefore, if an attempt is made to obtain a ferromagnetic iron oxide powder having a high coercive force using this method, the defects of the cobalt ferrite layer grown on the surface of the ferromagnetic iron oxide powder appear gradually. Hence, it is difficult to improve the stability of the ferromagnetic iron oxide and to obtain a ferromagnetic iron oxide powder having a high coercive force.
In the method which comprises adhering a cobalt compound to the surface of cobalt-free ferromagnetic iron oxide, the coercive force attainable is at most about 500 to 600 Oe, and it is difficult to obtain higher coercive forces which are required for modern magnetic recording media.
A method to increase the coercive force further by adding ferrous ion (Fe.sup.+2) together with the cobalt compound (see, for example, Japanese Patent Publication Nos. 24237/77 and 36751/77) is also known. However, products obtained using this method have poor dispersibility in organic solvents, and therefore, magnetic recording media prepared using the ferromagnetic powder obtained by this method have the defect that a large amount of noise is developed. For use in high density recording, therefore, an improvement has been desired on this situation.